Studies have been conducted in the past into the application of a solid electrolyte having ion conductivity, such as stabilized zirconia, lithium nitride, or β-alumina, to the electrolytic membranes of fuel cells, the electrolyte used in completely solid cells, sensors, and so forth. The ion conductivity of these solid electrolytes is believed to be the result of the movement of ions through the lattice or defects of a solid.
Also, there are polymer electrolytes such as perfluorosulfonic acid or hydrocarbon-based polymers, which have been studied as electrolytic membranes for solid polymer type fuel cells, and it is known that when these are wetted with an electrolytic solution such as water, ions move through the electrolytic solution present in the voids of the polymer chain, resulting in ion conductivity. These are called quasi-solid electrolytes. Among the advantages to these polymer electrolytes are that they exhibit ion conductivity at a relatively low temperature, they are easy to mold into thin films and so forth, and they provide good contact with electrodes, and for these reasons they are very promising as electrolytic membranes for fuel cells.